After some testing i can confirm there is a bug, or two. But it is caused by some other thing. I have changed the description of the ticket 
SPARK-1569.
In general, there is a problem in Spark when no chat windows is opened and it receives a new message through already established OTR session. The first message will be gibberish as OTR module is not loaded at that point. It is only loaded if you open the chat window before getting the message. Some racing condition bug which also causes this SPARK-1548.
Your workaround will only work if the starter of OTR session closes the window. When the other side closes the window OTR session is still not closed.
As we have no active Spark developers and OTR sessions is rather complex thing, i doubt this will be fixed anytime soon..